1. Field of the Invention
The present invention relates in general to the manufacture of a field-effect transistor epitaxial structure. More particularly, the present invention relates to a novel material system and epitaxial structure for a modulation-doped field-effect transistor (MODFET) or lattice-matched and pseudomorphic high electron mobility transistors (HEMTs and pHEMTs).
2. Description of the Related Art
A MODFET is a field-effect semiconductor transistor designed to allow electron flow to occur in an undoped channel layer so that the electron mobility is not limited by impurity scattering. MODFETs are used in a variety of electronic devices such as solid-state power amplifiers, low-noise amplifiers as well as satellite receivers and transmitters, advanced radar and fiber-optics operating in microwave and millimeter wave systems.
A conventional MODFET includes a GaAs substrate, a buffer layer, a channel layer, a spacer layer, a donor layer, a barrier layer (also known as a Schottky layer) and a cap layer formed on the substrate. The barrier layer may be doped to function as both the Schottky barrier layer as well as the donor layer. The spacer, donor and barrier layers are typically formed of a wide bandgap semiconductor material such as aluminum gallium arsenide (AlGaAs). The channel and cap layers are formed of a narrow bandgap semiconductor material such as gallium arsenide (GaAs) or gallium indium arsenide (GaInAs).
A lattice-matched high electron mobility transistor (HEMT) is a type of MODFET, having a narrow-gap semiconductor material lattice-matched to the wide-gap semiconductor material. A pseudomorphic high electron mobility transistor (pHEMT) is another type of MODFET in which the narrow-gap semiconductor material is not lattice-matched to the wide-gap semiconductor material. For example, a conventional pHEMT based on the Al.sub.a Ga.sub.1-a As/Ga.sub.x In.sub.1-x As material system has a typical profile employing a=0.3 in the donor layer and x=0.2 in the channel layer.
In these devices, discontinuity in the energy gaps between the wide-gap semiconductor donor layer and the narrow-gap semiconductor channel layer causes electrons to remain in the channel layer, but very close to the heterojunction because of the electrostatic attraction from the donor atoms. Conduction of electrons therefore takes place in an undoped channel layer so that the electron mobility is not limited by impurity scattering.
Manufacturers have attempted to create HEMT devices having a channel layer with a large sheet charge density which result in devices with higher current-carrying capabilities. Increasing the conduction band discontinuity (.DELTA.Ec) between the donor layer and the channel layer increases the sheet charge concentration in the channel layer of a conventional pHEMT based on the Al.sub.a Ga.sub.1-a As/Ga.sub.x In.sub.1-x As material system. However, increasing the Al percentage compromises the reliability of the device. In addition, a high-Al AlGaAs can also be chemically unstable.
Furthermore, for power transistor applications, it is desirable to decrease the hole injection from the channel layer into the barrier layer, thus increasing the gate-drain breakdown voltage and reducing the gate current. A high valence band discontinuity (.DELTA.Ev) between the channel layer and the barrier layer reduces the hole injection into the barrier layer. Conventional devices often employ a double gate recess construction to obtain a high gate-drain breakdown voltage. Fabrication of the double gate recess, however, adds additional steps to the manufacturing process thus increasing the complexity and cost of manufacturing the device.
It is known in the art that incorporating a greater proportion of In in the Ga.sub.1-x In.sub.x As channel layer improves the performance of the pHEMT. However, the amount of In that can be added to the channel layer is limited because an increased proportion of In causes a lattice strain buildup in the channel layer. Though the strain buildup of the channel layer can be compensated, it is difficult to use AlGaAs as a strain-compensating layer because AlGaAs is basically lattice-matched to the GaAs substrate.
Moreover, a high-resistivity, wide bandgap material is desirable to improve the pinch-off characteristics of the HEMT or pHEMT.